Electric arc furnace having side-wall lining protection

ABSTRACT

An electric arc furnace has a side wall lining with at least one portion bombarded by the arc flare from one of the furnace arcing electrodes. A magnet is positioned on the outside of this portion and ferro-magnetic particles are fed to its inside, the magnet forming a magnetic flux field traveling through this lining portion and on the latter&#39;&#39;s inside magnetically holding the particles to form an arc flare shield protecting this portion&#39;&#39;s inside from the arc flare. As the particles heat to their Curie point temperatures, they become non-magnetic and fall into the melt of the furnace, a supply of fresh and relatively cold particles being fed to the flux field as required to maintain the effectiveness of the arc flare shield formed by the particles. The wall thickness of this portion is reduced from its outside inwardly to form a recess in its outside and in which the magnet is positioned to reduce the distance the flux field must travel to the inside to hold the ferro-magnetic particles. Between the magnet and this thin wall portion a water cooled plate is positioned to cool the thin wall portion of the lining. In this way the travel path of the magnetic flux is reduced in length without risking possible overheating of the lining, permitting the use of a less powerful magnet construction, this being an electromagnet and requiring electric solenoids, a core or cores with pole pieces and a suitable mounting arrangement. A temperature measurement sensor is installed in the thin wall of the lining portion protected by the flare shield, and equipment is provided for controlling the feed rate of the ferro-magnetic particles which form the shield, automatically in response to the temperature measurements obtained from this sensor, this assuring maintenance of the effectiveness of the shield. The water-cooled plate may be made of non-magnetic metal to avoid interference with the flux travel through the plate.

United States Patent [191 Hanas et a1.

[ 1 ELECTRIC ARC FURNACE HAVING SIDE-WALL LINING PROTECTION [75]Inventors: Bertil Hanas; Jan Marchner, both of Vasteras, Sweden [73]Assignee: Allmanna Svenska Elektriska Aktiebolaget, Vaster'as, Sweden[22] Filed: Apr. 5, 1974 [2]] Appl. No.: 458,353

[30] Foreign Application Priority Data Apr. 5, 1973 Sweden 7304776 [52]U.S. Cl 13/35; 13/9 [51] Int. Cl. F27d 1/00 [58] Field of Search 13/9,35

[56] References Cited UNITED STATES PATENTS 3,619,467 11/1971 Goodman13/35 3,743,752 7/1973 Furuhashi 3,777,043 12/1973 ONeill 3,829,5958/1974 Nanjyo et al. 13/35 X Primary Examiner-G. Harris Attorney, Agent,or Firm-Kenyon & Kenyon Reilly Carr & Chapin [57] ABSTRACT An electricarc furnace has a side wall lining with at least one portion bombardedby the arc flare from one of the furnace arcing electrodes. A magnet ispositioned on the outside of this portion and ferro- May 13, 1975magnetic particles are fed to its inside, the magnet forming a magneticflux field traveling through this nace, a supply of fresh and relativelycold particles being fed to the flux field as required to maintain theeffectiveness of the arc flare shield formed by the particles. The wallthickness of this portion is reduced from its outside inwardly to form arecess in its outside and in which the magnet is positioned to reducethe distance the flux field must travel to the inside to hold theferro-magnetic particles. Between the magnet and this thin wall portiona water cooled plate is positioned to cool the thin wall portion of thelining. In this way the travel path of the magnetic flux is reduced inlength without risking possible overheating of the lining, permittingthe use of a less powerful magnet construction, this being anelectromagnet and requiring electric solenoids, a core or cores withpole pieces and a suitable mounting arrangement. A temperaturemeasurement sensor is installed in the thin wall of the lining portionprotected by the flare shield, and equipment is provided for controllingthe feed rate of the ferro-magnetic particles which form the shield,automatically in response to the temperature measurements obtained fromthis sensor, this assuring maintenance of the effectiveness of theshield. The water-cooled plate may be made of non-magnetic metal toavoid interference with the flux travel through the plate.

ELECTRIC ARC FURNACE HAVING SIDE-WALL LINING PROTECTION BACKGROUND OFTHE INVENTION An electric arc furnace conventionally comprises a shellcontaining a non-metallic refractory lining, and it has a cover or roofthrough which electrodes pass with lower ends reaching down to form arcsbetween the lower arcing ends of the electrodes and a metal in thebottom or hearth portion of the lining. Although other electrodearrangements are possible, when the furnace is powered by three-phasealternating current, there are three electrodes usually arranged intriangular formation. This positions the electrodes close enough to thefurnace side wall lining above the melt, so that the adjacent portionsof the lining are bombarded by the arc flares. These flares are in theform of both radiation and particles which bombard the portions of theside wall lining adjacent to the arcs. If the electrodes are positionedto form a straight line of electrodes, it is the flares from the endelectrodes which bombard the adjacent portions of the side wall lining.

Any portion of the side wall lining bombarded by an arc flare, issubjected to more heat and more rapid erosion and wear than the balanceof the lining. It is, of course, undesirable to have portions of thelining rendered unserviceable prior to the balance of the lining.

One proposal to protect such bombarded portions of an electric arcfurnace side wall lining, is disclosed by the Daniel J. Goodman US. Pat.No. 3,619,467, dated Nov. 9, 1971, the disclosure of which is herebyincorporated into the present disclosure, by this reference to thatpatent. The Abstract of that patent is quoted below:

Damage to the lining of the shell of an electric arc furnace by arcflare during steel making is reduced by providing a powerful magneticfield in the vicinity of the lining sidewall area subjected to theintensive heat from the arc flare and then feeding pieces, pellets orfragments of magnetically attracted ferrous material, such as iron oreor metal pieces, pellets or fragments past the magnetized lining area tothe molten metal and slag bath in the bottom or hearth of the furnace.This magnetic flux causes the falling fragments, such as concentratediron ore pellets, to be captured by and to adhere temporarily to thelining of the furnace until their temperatures reach the so-called Curiepoint of about 700C. at which their capability of being magneticallyattracted ceases. Thereupon the pellets fall into the molten bath butare immediately replaced by fresh pellets falling past the magnetizedwall and thus providing a continuous shield of pellets or othermagnetizable fragments which provide substantial protection to thesidewalls or lining of the furnace. The portions of the furnace betweenthe electromagnets providing the magnetic flux and the furnace lining atthe locations subjected to damaging arc flare are preferably formed ofnon-magnetic material, so as to maintain the magnetic flux flow throughthe furnace wall and lining at its maximum value.

In the above proposal, the magnetic flux produced by the electromagnetsmust travel through the thick nonmetallic refractoryside wall liningbefore forming the flux field useful for holding the magnet particlesforming the flare shield. This makes it necessary to use undesirablylarge electromagnet assemblies and, of course, requires an undesirablyhigh consumption of current. Even so, as indicated by the patent, themagnetic particles must be in the form of pellets or fragments, such asconcentrated iron ore pellets. It has been considered that concentrateiron ore in powdered form cannot be used satisfactorily.

Another prior art approach to protecting the bombarded portions of thefurnace side wall lining, has been to apply water cooled plates to theoutside of the metal furnace shell which conventionally contains thefurnace lining, the object being to in this way remove heat from theaffected lining portions more rapidly through the lining and the metalshell. An example of this is shown by Swedish patent specification328,599. However, with large electric arc furnaces using powerful arcs,the improvement effected leaves further improvement desirable.

SUMMARY OF THE INVENTION The present invention may be regarded as animprovement on the patented proposal using the electromagnets on theoutside of the furnace side wall lining, at the bombarded portions, inconjunction with the feed of ferro-magnetic particles into the fluxfields thus created on the insides bombarded by the arc flares.

This improvement comprises the formation of a recess in the outersurface of each affected furnace side wall lining portion, so that thisportion has a reduced wall thickness as compared to the balance of thelining. In this way the travel path of the flux through the lining isreduced in extent, permitting the use of smaller electromagnetstructures to provide a flux field on the inside for holding themagnetic particles effectively and, of course, a reduction in theelectric power consumption that was previously required. To protect thiswall portion of reduced thickness against the possible risk ofover-heating, a water-cooled plate is interposed between theelectromagnetic structure and this wall portion and in contact with thelatters inside. The thickness of this plate need not be very great,permitting the electromagnet structure pole piece to be positionedinside of the recess and thus substantially closer to the inside of thebombarded lining portion.

To further protect the bombarded side wall portion from overheating, atemperature sensor is built into this portion and connected to a feedrate controller for the feed of ferro-magnetic particles. This is donein such a way as to make the feed rate of the particles into themagnetic field on the inside of the furnace lining side wall, increaseas the temperature of the thinned wall portion increases, and todecrease when the temperature drops below a value indicating that thewall portion is at lower temperatures. As the ferro-magnetic particlesare heated to their Curie points, become nonmagnetic and fall, new freshparticles of lower temperatures are thus fed as replacements. With thethickness, and therefore protective effectiveness, of the flare shieldmaintained at a substantially constant value, the portion of reducedwall thickness can be relied on to have a service life substantially aslong, if not as long, as the balance of the furnace lining.

With this invention the magnetic flux path length through the lining canbe very substantially shortened, and with an electromagnetic structureor assembly of the size and power as formerly required, a very intensiveflux field of great force may be maintained on the inside of the liningwhere the protection is needed against the arc flare. This makes itpossible to use a feed of powdered concentrated iron ore for thecreation and maintenance of the arc flare shielding, althoughconcentrated iron ore in the form of pellets or fragments of substantialsize, as well as ferro-magnetic metal pieces, such as steel scrapparticles, may be used, but such use is not to be considered mandatoryas it was before.

The water-cooled plate may be made of nonmagnetic metal to avoidinterference with the flux travel.

BRIEF DESCRIPTION OF THE DRAWING The presently preferred mode ofcarrying out the invention is schematically illustrated by theaccompanying drawing in which the single view is a vertical section ofan electric arc furnace to which the invention is applied.

DETAILED DESCRIPTION OF THE INVENTION Referring to the above drawing,the furnace vessel is indicated as having a lower portion or hearth 1and a side wall 2 which extends upwardly from this portion 1, aremovable cover 3 covering the top of the vessel. Al-

' though not shown because of its familiarity in the prior art, thefurnace vessel normally has a steel shell on its outside with theportions 1 and 2 formed by a nonmetallic, refractory lining, only thislining being shown by the drawing. Cantilever arm 4 mounts theelectrodes 11 which are suspended through openings in the cover 3.Although shown for only one of the electrodes, each of the electrodesforms an arc 11a, the extremely hot lower end portion of the electrodeproducing the arc flare 12 consisting of thermal radiation and possiblyparticles, the intensity of the bombardment action depending on thepower applied to form the arc 11a. The arcs are, of course, between thebottom ends of the electrodes and the metal melt 13 in the lower orhearth portion 1 of the vessel. The bombarded portion of the side wall,of which there are more than one, is indicated at 14 as being formed byrefractory bricks from which the entire side wall lining 2 may also beformed. The inside surface of this portion 14 is flush with thegenerally cylindrical plane of the inside of the furnace side walllining 2.

However, on the outside of this bombarded portion, the furnace lining isformed with a recess 14a so that this portion 14 has a reduced wallthickness. As indicated by the drawing, this reduced wall thickness isvery substantially thinner than the balance of the side wall lining 2.For its protection against possible overheating, its inside is incontact with hollow watercooled cooling plate or box 15. A continuouscirculation of cooling water is maintained in this plate or box 15 asindicated by the arrows A-A. For illustrative reasons, this plate or box15 is shown as being possibly somewhat thicker than should be used; theplate or box 15 takes up as little space in the horizontal direction asis possible, consistent with adequate cooling of the thinned wallportion 14.

The recess 14a and the relative thinness of the side wall portion 14 andof the plate or box 15, permit the pole pieces 16 and 17 of anelectromagnet assembly, to be positioned in the recess to provide theshortened flux travel previously described. These pole pieces are shownas provided with electrically powered solenoids 16a and 17arespectively. Prior art knowledge of electromagnet design may be used,but the pole piece ends which create the flux field should be positionedinside of the recess 14a, with the flux path travel distance shortenedas much as possible. The furnaces steel shell should have openings orcutouts to provide the necessary clearance for the recess 14a, and forinstallation of the water-cooled plate 15, and insertion of the polepiece ends, if not the major part of the electromagnet assembly, in therecess 14a.

The feed for the ferro-magnetic particles which in the case of thisinvention may be powdered concentrate iron ore as well as iron orepellets or fragments, is through a suitable feeder 18 in the furnacecover 3. There may be a plurality of these feeders, or feeders in theform of circular segments may be used. The feed of the material is underthe control of a feed controller 19 which may be an electricallycontrollable valve. With the controllable valve or feed controller, thetemperature sensor is shown at 20 as controlling this valve orcontroller 19 via a proportional or proportional integrating type ofamplifier 21. The sensor 20 may be of the type disclosed by the Otto vonKrusenstierna et al US. Pat. No. 3,512,413, dated May 19, 1970. Thecontrol circuitry is such that the feed through the feeder 18 increasesif the wall temperature of the portion 14 increases, with the feed ofparticles decreasing if the wall temperature at 14 decreases. In thisway the supply of ferro-magnetic particles, required to replaceparticles heated above their Curie points and falling, is madeautomatically responsive to the wall temperature of the portion 14,assuring the maintenance of an arc flare shield adequate to protect thethinned portion 14 against overheating; this protection is in additionto that afforded by the cooling plate or box 15. If powdered iron oreconcentrate is used, this feed, via the feeder or duct 18, may bereinforced by a stream of carrier gas of suitable composition and underthe pressure of a compressor or blower (not shown), in which case,assuming the compressor or blower is powered by an electric motor, themotor speed may be varied under the control of the sensor 20.

Because of the possibility of providing a very strong flux field in theinside of the wall portion 14, a relatively high speed stream of theferro-magnetic particles in powdered form, for example, can be used toadvantage. The magnetic adhesive force drawing the particles against thewall portion 14 in its inside, may be made very intense.

The foregoing reference to Curie points is to be understood as meaningthat for the composition of the ferro-magnetic material used, thisreference is intended to mean the temperature or temperatures wherethere is change from a magnetic phase to a non-magnetic phase. Underpractical furnace operating conditions, the points can be detectedbecause the particles will fall into the melt and require replacement.It is well known that the length of the path through which magnetic fluxmust travel determines the intensity or strength of the flux field atthe location where this field is to do useful work, which in the presentinstance is the attraction and holding of the ferro-magnetic particles.

As explained by the previously referred to patent, any metal between themagnet and the lining inside, should preferably be non-magnetic. Withthis in mind, the water-cooled plate or plates 15 may be made ofnon-magnetic metal.

What is claimed is:

1. An electric arc furnace comprising at least one electrode having anend forming an are producing an arc flare, a side wall lining having atleast one portion having an inside positioned to be bombarded by saidflare, said portion having an outside, a magnet having pole-piece endson said outside forming a magnet flux field traveling through saidportion and beyond said inside and on said inside holding ferro-magneticparticles forming a flare shield until the particles are heated to theirCurie point temperatures, and a feeder for feeding said field withreplacement ferro-magnetic particles at temperatures lower than theirCurie point temperatures; wherein the improvement comprises said lininghaving an outer surface in which a recess is formed at said portion sothat said portion has a reduced wall thickness through which said fluxfield must travel, and

means between said magnet and recess for watercooling said portion ofreduced wall thickness.

2. The furnace of claim 1 in which said flare shield is formed by ironore particles in powder form.

3. The furnace of claim 1 in which said magnet pole piece ends areinside of said recess.

4. The furnace of claim 3 in which said means is a water-coolednon-magnetic plate positioned against said outside between the latterand said magnet pole piece ends.

5. The furnace of claim 4 in which said feeder has a feed ratecontroller and means for actuating said controller automatically inresponse to the temperature of said portion.

1. An electric arc furnace comprising at least one electrode having anend forming an arc producing an arc flare, a side wall lining having atleast one portion having an inside positioned to be bombarded by saidflare, said portion having an outside, a magnet having pole-piece endson said outside forming a magnet flux field traveling through saidportion and beyond said inside and on said inside holding ferro-magneticparticles forming a flare shield until the particles are heated to theirCurie point temperatures, and a feeder for feeding said field withreplacement ferro-magnetic particles at temperatures lower than theirCurie point temperatures; wherein the improvement comprises said lininghaving an outer surface in which a recess is formed at said portion sothat said portion has a reduced wall thickness through which said fluxfield must travel, and means between said magnet and recess forwater-cooling said portion of reduced wall thickness.
 2. The furnace ofclaim 1 in which said flare shield is formed by iron ore particles inpowder form.
 3. The furnace of claim 1 in which said magnet pole pieceends are inside of said recess.
 4. The furnace of claim 3 in which saidmeans is a water-cooled non-magnetic plate positioned against saidoutside between the latter and said magnet pole piece ends.
 5. Thefurnace of claim 4 in which said feeder has a feed rate controller andmeans for actuating said controller automatically in response to thetemperature of said portion.